Process for the treatment of anodic oxidized aluminum surfaces

ABSTRACT

AN IMPROVEMENT IN THE TREATMENT OF ANODIC OXIDIZED ALUMINUM SURFACES WITH SUBSEQUENT SEALING WITH HOT WATER OR STEAM. THE IMPROVEMENT COMPRISES EITHER AN INTERMEDIATE TREATMENT OR A COMBINED TREATMENT WITH THE SEALING OF THE ALUMINUM SURFACES AT FROM 15*C. TO 95*C. WITH A SOLUTION OF ACRYLIC ACID AND/OR A POLYMER OF ACRYLIC ACID, METHACRYLIC ACID OR MALEIC ACID HAVING A SPECIFIC VISCOSITY OF UP TO N=0.75 CP. IN A 0.7% SOLUTION IN 2 N NAOH AT 20*C. THE TREATMENT PREVENTS THE FORMATION OF SEALING FILMS WITHOUT IMPAIRING THE ANODIC OXIDE COATING OR THE QUALITY OF THE AFTER-SEALING.

United States Patent U.S. cl. 204-35 N g 5 Claims ABSTRACT OF THE DISCLOSURE An improvement in the treatment of anodic oxidized aluminum surfaces with subsequent sealing with hot water or steam. The improvement comprises either an intermediate treatment or a combined treatment with the sealing of the aluminum surfaces at from 15 C. to 95 C. with a solution of acrylic acid and/or a polymer of acrylic acid, methacrylic acid or maleic acid having a specific viscosity of up to 1,=0.75 cp. in a 0.7% solution in 2 N NaOH at 20 C. The treatment prevents the formation of sealing films without impairing the anodic oxide coating or the quality of the after-sealing.

THE PRIOR ART Anodic oxide coatings are frequently applied to aluminum surfaces for the purpose of corrosion protection. These oxide coatings protect the aluminum surface against the effects of the atmosphere and other corrosive media. Furthermore, anodic oxide coatings are also applied so as to obtain a harder surface and thus improve wear resistance of the aluminum. By virtue of the color of the oxide coatings or their readily partial colorability, particularly decorative effects can be achieved.

A number of processes are known for the production of anodic oxide coatings on aluminum. The production of the oxide coatings takes place, for example, by direct current in solutions of sulfuric acid (direct current-sulfuric acid process). Frequently, however, solutions of organic acids, such as, in particular, sulfophthalic acid or sulfanilic acid or mixtures of these acids with sulfuric acid are used. The last mentioned processes are known, in particular, as hard anodizing processes.

These anodically applied oxide coatings do not fulfill all requirements in respect of corrosion protection, however, since they provide a porous structure. For this reason it is necessary subsequently to seal the oxide coatings. This after-sealing is usually carried out with hot or boiling water or steam and is known by the name of sealing. In this way the pores are closed and thus the corrosion protection is substantially enhanced.

In the after-sealing of anodically applied oxide coatings not only the pores are sealed, however, but a velvety film of greater or lesser thickness, the so-called sealing film, forms over the entire surface. This film consists of amorphous aluminum hydroxide and is not resistant to handling, so that the decorative effect of the coating is impaired. For this reason it has been necessary until now to remove this film mechanically by hand.

OBJECTS OF THE INVENTION It has now been found that these disadvantages can be substantially prevented if the process described below for the treatment of the surface of aluminum alloys by anodic oxidation with a subsequent sealing by steam is used.

An object of the present invention is the development in the process for treating aluminum surfaces which comprises subjecting aluminum and aluminum alloy surfaces to an anodic oxidation and subsequent sealing with hot water or steam, the improvement selected from the group consisting essentially of (A) applying a solution from 0.01 to 10 gm. per liter of an acid selected from the group consisting of (1) acrylic acid at a pH of from 5 to 6 and (2) a polymer of acrylic acid, a polymer of methacrylic acid, a polymer of maleic acid, copolymers of maleic acid with ethylene or vinyl alcohol, and mixtures thereof, said polymers having a specific viscosity of up to 0 :0.75 cp. in a 0.7% solution in 2 N NaOH at 20 C., to the anodic oxidized surfaces at temperatures of from 15 C. to C. prior to said sealing and (B) applying said solution to the anodic oxidized surfaces simultaneously with said sealing with hot water.

These and other objects of the invention will become more apparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION The present invention, therefore, provides a process for the treatment of aluminum or aluminum alloy surfaces by anodic oxidation with a subsequent sealing by water or steam in which prior to the completion of the sealing, the said surfaces are treated at a temperature of from 15 C. to 95 C. with a solution containing acrylic acid and/or a polymer of acrylic acid, methacrylic acid or maleic acid having a specific viscosity of up to 1 :0.75 cp. in a 0.7% solution in 2 N NaOH at 20 C.

It is preferred that the said intermediate treating solution contains from 0.01 to 10 gm./liter of acrylic acid and/or a polymer of acrylic acid, methacrylic acid or maleic acid.

In this process, acrylic acid and polymers of acrylic, methacrylic and maleic acid can be used. The polymers of maleic acid may be pure polymers or copolymers, for example, with ethylene or vinyl alcohol. It has been found appropriate to use polymers Whose specific viscosity does not exceed a value of 0.75 cp. The specific viscosity measured is always that of a 0.7% solution in 2 N NaOH at 20 C. in a Brookfield rotary viscometer.

Preferably the intermediate treatment is carried out with solutions containing polyacrylic acid and/or polymethacrylic acid with viscosities of from 1 :0.05 to 0.2.5 cp.

The solution for the intermediate treatment preferably contains 0.1 to 10 gm./liter of the said compounds which may be used singly or in a mixture, though it is also possible to use larger quantities,,but this brings no further advantages.

The above described solutions for the intermediate treatment preferably have a pH value situated between 5 and 6. This pH value has proved appropriate, particularly with the polymers in that an additional pH adjustment is not required. Such a pH value should be adjusted to this value however, if for the preparation of as potassium, sodium or ammonium salts of the acids are used."' In"general, sucha'procedure is not recommended since as a result neutral salts enter into the solutions. It is more advantageous, in fact, to prepare the solutions with the free acids by using fully demineralized or distilled or condensed water.

The intermediate treatment is carried out at temperatures between 15 C. and 95 C., preferably at temperatures of over 50 C., the anodized articles of aluminum or aluminum alloys being either dipped into solutions or are sprayed with the same. The duration of treatment is from 0.5 to 60 minutes but generally under minutes. Longer treatment times; however, have no detrimental eifect. After the intermediate treatment with the solutions described above, rinsing with water may be eifected to avoid an entrainment of substances which are removable by rinsing in the subsequent sealing stage. This rinsing is not essential, however, and no disadvantages result if the aluminum parts are transferred, immediately after the intermediate treatment, to the aftersealing.

\In a particular embodiment of this process the aftersealing with water and the intermediate treatment are carried out in one operating stage. In this case the water provided for the after-sealing is mixed with the said compounds in the specified quantities and the sealing is carried out in the customary manner.

The'after-sealing solutions may also contain additives known for such purposes, such as nickel acetate, in small quantities. It is possible by the new process to prevent the formation of sealing films without impairing the anodic oxide coating or diminishing the quality of the after-sealing. The appearance of the surface is not affected by the process in accordance with the invention. The effects, as achieved by pretreatment and anodizing, remain unchanged.

The following examples are illustrative of the prac-' tice of the invention without being limitative in any manner.

In the following examples the quality of theoxide coatings is determined by the so-called Testal value according to DIN 50 949. Values which according to this test are above are technically not satisfactory. The designation of the aluminum alloys in the examples is in accordance with DIN 1725. DIN is the abbreviation for "Deutsche Industrie-Norm representing a series of standard German published test procedures.

EXAMPLE 1 Aluminum shaped specimens (AlMgSi0.5), alkaline degreased and pickled in the customary manner, were anodically oxidized by the hard anodizing process (thickness of coating 29 and treated with a solution of 5.0 gm./liter of polyacrylic acid (1 =0.08 cp. in a 0.7% solution in 2 N NaOH at C.) for 10 minutes in fully demineralized water at 80 C. After an intermediate rinsing the aluminum parts were sealed in hot water (100 C., 60 minutes). After which, the specimen showed no sealing film. The thickness of coating after sealing was 29 the Testal value had dropped from over 200 to 7.5. I

EXAMPLE 2 Aluminum shaped specimens (AlMg3) degreased in the customary manner and anodically oxidized by the direct current-sulfuric acid process (thickness of coata -EXAMPLE 3 r. e s S), a kal n decr as and pickled in the customary manner and anodically oxidized by the direct current sulfuric acid-oxalic acid process (thickness of coating 15;), were sealed in a 1.0 gm./ liter of polymethacrylic acid (1 =0.10 cp. in a 0.7% solution in 2 N NaOH at 20 C.) solution in fully demineralized water for 60 minutes at 100 C. The sheets showed no sealing film. The thickness of coating after the sealing was 1511.; the Testal value had dropped from over 200 to 10.0.

EXAMPLE 4 Aluminum shaped specimens (AlMgSi0.5), alkaline degreased and pickled in :the' customary mannerand anodically oxidized by the direct current sulfuric acid process (thickness, of coating 20 to.30;i), were sealed. in a solutionwhich'contained 5 gm./liter of the chemicals listed in the following table 'and which had 'been"adjusted to a pH value of 5 to 6 for 60 minutes at 100 C., or they were pretreatedin the solution for 10 minutes at C. and after an intermediate rinsing sealed in hot water at C. for'60 minutes. .The thickness of the coating remained unchanged on the different specimens. The formation or prevention of the sealing film and the varying quality of the sealing, determined on the basis of the Testal value, are summarized in the following table. Only when treatment in accordance with the present invention was carried out, a sealing film did TABLE Testal value Chemicals present in the (DIN 50949) intermediate treatment after sealing Sealing film Tartaric acid 200.0 Not prevented. Methacryli acid.-- 7.5 Do. Itaeonie acid---" 9.0 -Do. Polyacrylic acid (1;=0.9 in a 0.7% 25.0 Partly presolution in 2 N NaO at 20C.).- vented. Sodium gluconate 70. 0 Prevented. Polyacrylic acid (1 =0.35 cp. in a 0.7% 10. 0 D0.

solution in 2 N NaOH at 20 0.). Copolymer of maleic acid and vinyl 8.0 Do.

alcohol (1 =0.18 cp. in a 0. %..solution in 2 N NaOH at 20 0.). Copolymer of maleic acid and ethyl- 9.0 D0.

ene (11=0.12 cp. in a 0.7% solution x 1 in2NNa Hat20 C.).

Acrylic acid'. I i

faces to an anodic oxidation and subsequent sealing withhot water or steam, the improvement consisting essentially of applying a solution containing from 0.01 to 1.0 gm; per liter of an acid selected from the group consisting of (1) acrylic. acidat a pH of'from 5 to 6" and 2) a polymer of acrylic acid, a polymer of methacrylic acid, a polymer of maleic acid, copolymers of maleic acid with ethylene or vinyfalcohol, and mixtures ,thereof,.jsaid polymers having a specific viscosity of up to 1 :0.75, cp. in a 0.7% solution in 2N NaOH at 20 C., to the anodic oxidized surfaces attemperatures of from 15 C. to 100 C. prior to completion of said sealing.

2. The process of claim 1 wherein said acid is -selectedfrom the group consisting of polyacrylic acid, polymethacrylic acid and mixtures thereof, having a specific viscosity of from 'r =0.05 to 0.25 cp.

3. The processof claim 1 wherein said application of said solution is at temperatures of over 50 C. v

4. The process of claim l-whereinsaid application of said solutions to said anodic oxidized surfaces is conducted at temperatures of from 15 C. to 95 C. prior to subsequent sealing for a period of about 10 minutes.

5. The process of claim 1 wherein said application of said solution is simultaneous with said sealing with hot water.

References Cited UNITED STATES PATENTS Oosterhout 117-19.8 Beck 117132 X 10 Emmons et a1 117-132 Topper 117-132 X Spooner 204--58 Riou et a1. 148-6.14 Waring 20438 E UX Knewstubb et a1.

204-38 E UX Cahne 204-38 E UX Kissin et a1. 204-58 X 6 9/1969 Anderson et al. 204-58 X 5/1945 Pullen 204-68 X 1/ 1962 Riou et a1. 204--35 X FOREIGN PATENTS 1 2/ 1953 Great Britain 204---38 E 5/ 19 39 Great Britain 20438 E OTHER REFERENCES The Surface Treatment of Aluminum. Wernick and Pinner, 1964, pp. 478-484, 486, 501, 506-509.

Honnylite Process, Honny Chemicals Co. Ltd, pp.

WILLIAM D. MARTIN, Primary Examiner 5 H. I. GWINNELL, Assistant Examiner U .8. Cl. X.-R. 

